The present invention relates generally to label printing and dispensing apparatus and relates more particularly to apparatus for the thermal printing of labels carried across a print head by a carrier web and thereafter dispensed from the web by passage of the web over a peeler bar.
Thermal transfer printing requires the precisely controlled feeding of the sheet being printed with respect to the print head heater elements known as the print line. Heat and pressure at the print line effect the transfer of waxed-based ink from the ink transfer foil to the paper surface.
For the printing of labels, the labels are conventionally presented to the print head in the form of spaced die-cut paper stock adhesively attached to a carrier web. The web is fed at a controlled rate across the print head by means of a motor driven roller which serves to pinch the web of labels and transfer foil against the print head with the print line being tangentially aligned with the roller. The label web is advanced by friction between the roller and the back surface of the web to provide a positive controlled drive of the web. The transfer foil, which travels with the web, acts as a sliding bearing surface against the smooth contact face of the print head.
Dispensing of the printed label from the web is accomplished by sharply changing the direction of travel of the web by passing the web over a sharp edge known as a peeler bar. This change of direction should be at least 100.degree. and a substantial web tension must be maintained during its passage over the peeler bar in order to properly dispense the label. Upon leaving the web, the label is conventionally transferred by air jet to an applicator head on which it is held by vacuum. The applicator head is then moved into engagement with the product to be labelled which typically is passing thereby on a conveyor.
To maintain the proper tension over the peeler bar, it is conventional for label printing machines to utilize a second motor driven drive roller engaging the web downstream of the peeler bar. Since the web feed rate must be the same at both the primary and secondary drive rollers to maintain proper web tensioning, an additional complication is presented by a two motor web feed system. Because of difficulties inherent in equalizing the drive rate at both drive rolls, it has been conventional to drive the second roll by means of a slip clutch to effectively maintain a predetermined web tension.
As indicated, web tension control is an important aspect of this type of printer applicator, not only for maintaining a predetermined web feed past the print head but also for insuring a proper peeling of the labels from the web at the peeler bar. The web tension requirements of conventional label printers has been such as to require a peeler bar of substantial dimensions to provide sufficient strength to withstand the web tension without deformation. It has not heretofore been practical due to physical size restraints to design a drive roll and peeler bar assembly characterized by a distance between the print line and peeler bar edge of less than approximately 3/4". This distance, which may be termed the "dead zone", is of particular importance since it determines the amount of label space which is not available for printing in certain situations. For so-called "on-demand" label printing, wherein each label is different, the information required for the printing of a given label may not be available before the previous label has been printed and dispensed. Even though the web drive is halted as the previous label clears the edge of the peeler bar, the leading edge of the succeeding label has already traveled past the print line and approached the peeler bar edge. To avoid sizeable non-printed areas of the labels, it would of course be possible to die-cut the labels to provide a space between labels substantially equal to the length of the dead zone. However, this solution wastes a substantial amount of label material, particularly in the case of small labels.